1. Field of the Invention
This invention relates to a production method of goethite which is useful as a raw material for magnetic materials such as .alpha.-iron powder, magnetite, .gamma.-Fe.sub.2 O.sub.3 and the like.
2. Description of the Prior Art
When processing a powdery raw material into a useful product, it has been well-known that properties stemming from the size, shape, crystallinity and the like of the particles play an important role. This also applies to magnetic materials. The size, shape, crystallinity and the like of goethite particles, which is a starting raw material, significantly affect the properties of a magnetic material to be obtained from the starting raw material. The control of the size, shape, crystallinity and the like of goethite particles has been the subject of a great deal of work. Among such physical properties of goethite particles, it has been considered to be the most difficult to control the particle size distribution.
For example, Japanese Patent Publication No. 21720/1977 discloses a production method of goethite in which ferrous hydroxide is vigorously agitated for at least several hours in a non-oxidizing atmosphere into a uniform system, followed by subjecting the thus-prepared uniform system to oxidation. However, it is difficult to convert a non-uniform floc containing ferrous hydroxide into a sufficiently uniform system by merely agitating the floc.
Another method is disclosed in Japanese Patent Laid-open No. 56196/1978 in which a non-uniform floc containing ferrous hydroxide is subjected to a neutralization reaction in the presence of a soluble silicate so as to make the floc uniform; and acicular goethite particles are then allowed to undergo a uniform growth reaction. However, when practicing the above method, it is necessary to use the soluble silicate in an amount as much as 0.1-1.7 atom % in terms of Si on the basis of Fe. Moreover, goethite takes the silicate therein and is thus converted into such a form as if diluted by the silicate. As a corollary to this, the magnetic properties of iron powder, which is to be obtained by reducing the above silicate-added raw material in a manner commonly known in the art, will be poor.
A further method is disclosed in Japanese Patent Laid-open No. 59095/1977 in which goethite is produced by controlling the oxidation velocity or rate of ferrous hydroxide, for example, in such a way that 8 wt.%, 25-55 wt.% and the remainder of ferrous hydroxide are oxidized in turn in a first stage of 0.5-4 hours, a second stage of 1.5-6 hours and a third stage of completing the oxidation of ferrous hydroxide in its entirety. In the above method, it is necessary to change the oxidation velocity in various ways in the course of the production process of goethite, thereby taking a relatively long time for the completion of the oxidation. In addition, it is also required to carefully control the oxidation velocity.
A still further approach is disclosed in Japanese Patent Laid-open No. 22637/1981 in which seed crystals, which have in advance been prepared at room temperature, are employed. This method, however, involves such a potential problem that more magnetite may occur than goethite unless the reaction conditions, particularly, the temperature conditions are precisely controlled.